1. Field of the Invention
The present invention relates to a resin composition for optical stereolithography, and a method for producing an optical stereolithographic product using the composition. More particularly, the invention relates to a composition for optical stereolithography containing a non-antimony-based cationic polymerization initiator which is excellent in safety and free from environmental pollution. Using a resin composition for optical stereolithography according to the present invention can safely and productively produce an optical stereolithographic product at a high stereographic speed, thus a product, which has low yellowness, high whole light transmittance, and color tone equivalent to or close to colorless transparency, and also exhibits excellent heat resistance, toughness, other mechanical properties, water resistance and the like, can be obtained at high curing sensitivity while preventing geoenvironmental pollution.
2. Description of Related Art
Various resin compositions for optical stereolithography have hitherto been proposed, for example, a composition containing a cation-polymerizable organic compound and a cationic polymerization initiator; a composition containing a radical polymerizable organic compound and a radical polymerization initiator; a composition containing a cation-polymerizable organic compound, a radical polymerizable organic compound, a cationic polymerization initiator and a radical polymerization initiator, or the like.
Among them in a resin composition for optical stereolithography containing a cation-polymerizable organic compound such as an epoxy compound, a cationic polymerization initiator existing in a system generates upon illumination cationic species (H+) which reacts with a cation-polymerizable organic compound such as an epoxy compound in a chain reaction, resulting in ring-opening of the cation-polymerizable organic compound, thus the reaction proceeds. Using a photo-curable resin composition based on a cation-polymerizable organic compound such as an epoxy compound generally results in a photo-cured product having better dimensional accuracy with little contraction, compared with those obtained using a photo-curable resin composition based on a radical polymerizable organic compound.
As a cationic polymerization initiator for photopolymerization of a cation-polymerizable organic compound, a cationic photopolymerization initiator is known, which initiator consists of an aromatic sulfonium salt of an element in group VIIa (see, patent Literature 1), an aromatic onium salt of an element in group VIa (see, patent Literature 2), an aromatic onium salt of an element in group Va (see, patent Literature 3), and the like. In a photo-curable resin composition containing a cation-polymerizable organic compound, a sulfonium salt containing antimony has been widely used as a cationic photopolymerization initiator.
However, an antimony compound requires extreme care for handling, because it is generally toxic and shows toxic effect like arsenic and mercury, and further has concerns about working- and global-environmental pollution.
In this regard, a resin composition for optical stereolithography has been proposed, which utilizes a cationic polymerization initiator consisting of a sulfonium salt containing phosphorous, instead of the conventionally widely used antimony-based cationic polymerization initiator (see, Patent Literatures 4, 5, 6).
However, when illuminated, a cationic polymerization initiator consisting of a sulfonium salt containing phosphorous generates an acid (cationic species) having lower acidity, compared with that obtained from an antimony-based cationic polymerization initiator, thus it has low catalytic activity as a cation polymerization initiator upon illumination, and often fails to provide a stereolithographic product with sufficient physical properties.
Under such circumstances, among phosphorous-based cationic polymerization initiators consisting of phosphorous-containing sulfonim salts, a phosphorous-based cationic polymerization initiator wherein an anionic part consists of a phosphate having a fluoroalkyl group is known to have an ability to initiate photopolymerization equivalent to those of the conventional antimony-based cationic polymerization initiator. However, when a phosphorous-based cationic polymerization initiator wherein an anionic part consist of a phosphate having a fluoroalkyl group is used to prepare a resin composition for optical stereolithography, and the resin composition for optical stereolithography is used to conduct optical stereolithography, the resulting stereolithographic product turns to pale yellow and a stereolithographic product has a color tone equivalent to or close to colorless transparency cannot be obtained. In particular, when a phosphorous-based sulfonium salt wherein an anionic part is a trifluorotris(fluoroalkyl)phosphate ion represented by the formula: [PF3(Rf)3−], wherein Rf is a fluoroalkyl, is used as a cationic polymerization initiator, the resulting stereolithographic product has problem of having enhanced yellowness and becomes gradually darker color when allowed to stand at room temperature.
A stereolithographic product obtained by optical stereolithography using a resin composition for optical stereolithography has been widely used, for example, as a model for validation of an appearance of various kinds of industrial products in designing them, a model to check the functionality of a component, a resin mold to form a casting mold, a base model to form a metal mold. Recently, there exists a need for a model having high transparency and free from yellowing such as a lens for an automobile or motorbike. Such a composition has also been utilized in the field of fine arts and crafts, for example, for restoration or simulation of art works, for contemporary art, as a presentation model for a design of a building covered with glass. Accordingly, as for a resin composition for optical stereolithography which contains phosphorous-based cation polymerization initiator excellent in safety, it is also required to produce a stereolithographic product having high transparency without yellowing.